The preparation of block copolymers is well known. In a representative synthetic method, an initiator compound is used to start the polymerization of one monomer. The reaction is allowed to proceed until all of the monomer is consumed, resulting in a living homopolymer. To this living homopolymer is added a second monomer that is chemically different from the first. The living end of the first polymer serves as the site for continued polymerization, thereby incorporating the second monomer as a distinct block into the linear polymer. The block copolymer so grown is living until terminated.
Termination converts the living end of the block copolymer into a non-propagating species, thereby rendering the polymer non-reactive toward monomer or coupling agent. A polymer so terminated is commonly referred to as a diblock copolymer. If the polymer is not terminated the living block copolymers can be reacted with additional monomer to form a sequential linear tri-block copolymer. Alternatively the living block copolymer can be contacted with multifunctional agents commonly referred to as coupling agents. Coupling two of the living ends together results in a linear triblock copolymer having twice the molecular weight of the starting, living, diblock copolymer. Coupling more than two of the living diblock copolymer regions results in a radial block copolymer architecture having at least three arms.
One of the first patents on linear ABA block copolymers made with styrene and butadiene is U.S. Pat. No. 3,149,182. These polymers in turn could be hydrogenated to form more stable block copolymers, such as those described in U.S. Pat. Nos. 3,595,942 and Re. 27,145. Various other block copolymers and processes for making them have been proposed over the years.
While block copolymers are often used in compounded form, the presence of certain of the typical blending components can also have a detrimental impact on properties. Common blending components include plasticizing oils, tackifying resins, polymers, oligomers, fillers, reinforcements and additives of all varieties. Oils are often added to such block copolymers to increase softness and improve processability to the compound. However, such oils also typically reduce the temperature resistance, strength and tear resistance of the compounds. What is needed now are new compounding materials that do not have such a dramatic negative effect on properties, while still imparting increased softness with improved processability.
Others have proposed compounds or articles having improved properties by blending in various other polymers. For example, Crossland et al, U.S. Pat. No. 3,766,295, suggests that compositions comprising 100 parts by weight of a block copolymer having at least two polymer blocks A of a monoalkenyl arene and at least one block of a hydrogenated diene when combined with 5-200 parts by weight of a low mol weight hydrogenated diene having a particular vinyl content have improved properties. However, as shown in the examples of Crossland, the only polymer that showed improvement in properties was a hydrogenated polybutadiene having a molecular weight of 9,100. Moreover, it is expected that blending such high mol weight, low polydispersity index homopolymers at high concentrations (greater than 100 phr) will result in oil bleed out at elevated temperatures. Korpman et al, U.S. Pat. No. 5,274,036 and related patents, discloses pressure sensitive adhesives comprising a solid rubber and a liquid rubber in a ratio of solid rubber to liquid rubber of 1:0.5 to 1:7. The liquid rubbers include hydrogenated polyisoprene and liquid hydrogenated polyisoprene where the liquid rubbers had a molecular weight from 10,000 to 75,000, although no properties were reported for materials of less than 25,000 molecular weight. Tg's of these liquid rubbers were typically <−55° C. However, it is difficult to handle the liquid rubbers separately from the solid rubber, and compositions were described only in adhesive formulations tested at body temperature or below.
Applicants have now discovered that, when certain low molecular weight anionic diene oligomers or polymers are combined in a particular way with the selectively hydrogenated block copolymers noted above, it is possible to obtain compounds which retain strength and upper service temperature properties, and also experience significant improvements in manufacturing steps and economies as well as improved properties such as increased softness without a significant reduction in processability. In addition, such compositions show little fogging and no smoke in film and fiber applications